<!DOCTYPE html>
<html>
<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
</head>
<body>
  <script>
    let ary = ['a', 'b', 'c'];
    let result = ary.includes('a');
    console.log(result);	// true
    result = ary.includes('e')
    console.log(result);	// false
  </script>
</body>
</html>